sometimes it doesn't boot [SOLVED]

[giobbino]

(Note: I suspect the issue I'm having have to do with fake ICs, especially fake Rockwell 65C02 / 65C22).

When I power on the board, usually does nothing, not even a black screen. It's totally out of signal, no raster. At the beginning I suspected it was something (spike?) related with the lack of a power-on switch, so I added one. No changes.

However I noticed If I give a little tap on the switch, removing the power to the board for a very little instant (enough for the LED to blink, however), often the computer boot or at least starts to boot: mostly it lead to a black screen (with raster) that can converted in a normal boot screen using a reset switch between pins #3 and #8 of the 555.

I also noticed that the same behavior happens with a fake MOS 6502 I've bought on Aliexpres for a couple of bucks (so this leads me to believe the Rockwell 65C02 is a fake too).

Any idea about how to solve this problem? BTW I've seen the decoupling capacitors are all 10 uF, while in most projects is used the 100 uF. Could it have something to do with that?

I've bought some more Rockwell ICs, but nobody grants they're legit and not (again!) fake. Unfortunately buying from Mouser or Digikey is not feasible; shipping to Brazil would cost more than 50 US$, because the haven't a cheap shipping option like Aliexpress.

[danwerner21]

I am not sure about the fake 65C02s -- I have not had that issue (I usually do not use aliexpress -- for that very reason).

There are a couple of other things you might check though. There are a few chips where logic family REALLY matters. I would make sure that U4 is a 7406 and not a 74LS06 or any other family. Same with U1 -- the 7402 needs to be a 7402. The last important one is U24 -- it needs to be a 74ALS32.

Checking those chips would be the first place I would start looking.

Dan

[giobbino]

Dan, thanks for your answer, it seems you pointed me in the right direction, albeit the results are different from the expected.

At first I did a test I should have done before: I checked with the scope the wave signal at the crystal pins... result, when the computer didn't boot there was a very strange wave frequency (13.2 MHz) or just some noisy wave at 38.2 Mhz). So I replaced the crystal and later also the variable capacitor. No changes.

Of course I used 7606, 73ALS32 and 7402. I got this last one from my working, fully socketed VIC-20 I uses for tests etc. - BTW The interesting part is that I replaced it with a 74LS02 and the VIC-20 still worked fine.

The 7402 I got from the VIC-20 had quite a recent date stamp; so I began wondering if it could be a fake CMOS labelled as 7402 as sometimes/often happens (however for some reason it worked into the VIC-20).

Since the VIC-20 worked fine with a 74LS02 in place of the 7402, I thought "why not?"... I got an old, legit 74LS02 and I put in the VIC2020.

Result: now the VIC2020 works fine, or at least it didn't fail anymore. So apparently a good 74LS02 in place of the 7402 can be used for the VIC2020. I will keep testing it and keep you informed.

My question is: what is the difference between 74 and 74LS ? AFAIK both should be NMOS, and LS was just a lower mA... and I can't find any different answer from the web.